Which of these numbers is prime? ${7,\ 87,\ 93,\ 98,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 7 are 1 and 7. The factors of 87 are 1, 3, 29, and 87. The factors of 93 are 1, 3, 31, and 93. The factors of 98 are 1, 2, 7, 14, 49, and 98. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 7 is a prime number.